Because Finasteride doesn't really block all DHT in serum (and even less in the follicle) and thats why it doesn't regrow.
This drug pathway is on the androgen receptors binding, so if it doesn't regrow, it is not super strong at that job. Even Dutasteride shows higher regrowth with just reducing serum DHT. Common sense. But maybe this is a good entry level for better mRNA pathways in future with higher efficacy.
Nonetheless good news to have more drugs acting on different pathways, so we can upgrade to a Big4, Big5... BigCope until HMI or haircloning is a real thing.